Expert: eigensteve - 6/27/2009

Question
while designing a car how is it tested in wind tunnel?what is the correct stream lined shape that must be designed to build a car?is the stream line shape necessary?

Answer
Hello Abdul,

This is an interesting question.  When designing cars, typically a scaled down model will be tested in a wind tunnel for aerodynamic efficiency.

The reason it is possible to use a scale model of the car is a scaling factor called the Reynolds number.  The Reynolds number is a dimensionless quantity obtained by three parameters:  a length scale L (such as width or height of the car), a velocity scale U (velocity of the car, or the wind in the tunnel), and viscosity of the fluid v (typically air).

R = LU/v

The idea is that, as long as the geometry and Reynolds number of two flows match, the flow structures (as well as lift and drag coefficients) will be identical.  This is called similarity, and is best illustrated with an example:

Say I have a beach ball of diameter 1m in a wind tunnel with velocity 1 m/s.  If I want to match the Reynolds number of this experiment with a ball that has diameter 1/10 m, I would need a wind tunnel velocity of 10 m/s.

This obviously neglects some important factors, such as the skin friction of the different balls, but it is a good starting place.


Now, on to the second part of your question: why is it important?  Aerodynamic efficiency is extremely important for automobiles, especially at high speeds and if they are to be fuel efficient.  Typically the wheel wells and side-view mirrors are difficult to streamline.  Also, it is important to keep the pressure drop from the nose of the car to the tail of the car as low as possible, or else we are wasting energy.

The aerodynamic problem is magnified with large cargo trucks.  If we could improve aerodynamic efficiency by a few percent on all of the trucks in America, this would translate into millions of dollars of savings on fuel and time.

Cheers,

Steve